Quasihydrodynamic approximation for memory functions in non-Markovian relaxation processes in condensed matter

A quasihydrodynamic approximation is introduced for senior memory functions in the chain of non-Markovian kinetic equations for time correlation functions in many-body systems. This approximation allows to describe complicated high-frequency and short wavelength spectra for a wide range of phenomena: from the molecular scattering and the scattering of slow neutrons in liquids to the description of the decay of magnetic induction signal in crystals.